40 
PALAEONTOLOGY OF NEW YORK. 
(Trenton); D. parvus, Whitfield, from the Galena limestone (Trenton) at 
Whitewater, Wisconsin; D. Schmidti, Davidson and King, is stated to be from 
the Lyckholmer Schichten of Esthonia, Russia, considered to be nearly the 
equivalent of the Caradoc and Trenton; the greatest individual development 
of the genus, however, is in the Niagara fauna and its equivalents, D. Com add 
being the American representative of D. Davidsoni, Salter, a Wenlock species 
occurring near Dudley and elsewhere in England, county Kerry, Ireland, and 
on the Island of Gotland; D. transversus and D. Woodwardi, Salter, are from 
the same horizon, and the Trematis Bohemica, Barrande (= Dinobolus ), is from 
an equivalent fauna (Etage E-e,). 
Genus MONOMERELLA, Billings. 1871. 
PLATE IV C, FIGS. 1-18; and PLATE IV d, FIGS. 1-15. 
1871. Monomerella, Billings. Canadian Naturalist, vol. vi, p. 220. 
1872. Monomerella, Davidson and King. Ann. and Mag. Nat. Hist.; Geol. Mag. ; Report Brighton 
Meeting of British Association. 
1874. Monomerella, Davidson and King. Quarterly Journal Geological Society, vol. xxx, p. 155. 
1875. Monomerella, Nicholson. Rept. Palaeontology of the Province of Ontario, p. 68. 
1875. Monomerella, Hall and Whitfield. Geol. Surv. Ohio; Palaeont., vol. ii, p. 131, pi. vii, figs. 1, 2. 
1884. Monomerella, Whiteaves. Palaeozoic Fossils, vol. iii, pi. 1, pp. 5, 6. 
Diagnosis. Shell usually thick, but in some cases quite thin; outline vary¬ 
ing from elongate-ovate to subcircular. Surfaces of contact of the valves some¬ 
times conspicuously broad. 
Pedicle-valve with a more or less elevated umbo, which may vary in height in 
a given species, in the type-species being high, as in Trimerella; umbonal cavity 
divided into two chambers by a longitudinal septum. Cardinal area large; 
deltidial ridges and deltidial callosities not always distinctly developed; cardi¬ 
nal slope usually well defined, merging anteriorly into the cardinal buttress, or 
umbonal septum; in some species this slope, or subtriangular area, is longi¬ 
tudinally divided by a furrow. The crescent is sharply impressed over the 
cardinal slope; terminal scars broad and distinct. Platform more or less 
developed; divided by the cardinal buttress, which extends nearly or quite its 
entire length. Muscular impression on the platform very large, covering the 
